1. Field of the Invention
The present invention relates to a black and white photothermographic material and an image forming method. More particularly, the invention relates to a black and white photothermographic material whose image tone is a preferable blue-black tone and which exhibits little change in color tone during image storage after a thermal developing process and an image forming method thereof.
2. Description of the Related Art
In recent years, in the medical field and the graphic arts field, there has been a strong desire for a dry photographic process from the viewpoints of environmental conservation and economy of space. Further, the development of digitization in these fields has resulted in the rapid development of systems in which image information is captured and stored in a computer, and then when necessary processed and output by communicating it to a desired location where the image information is output onto a photosensitive material using a laser image setter or a laser imager, and developed to form an image at the location on the photosensitive material. It is necessary for the photosensitive material to be able to record an image with high-intensity laser exposure and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using a pigment or a dye, such as ink-jet printers or electrophotographic systems, have been distributed as general image forming systems using such digital imaging recording material, images in the digital imaging recording material obtained by such a general image forming system are insufficient in terms of image quality (sharpness, granularity, gradation, and tone) needed for medical images used in making diagnoses and high recording speed (sensitivity). These kinds of digital imaging recording materials have not reached a level at which they can replace medical silver halide film processed with conventional wet development.
A photothermographic material using an organic silver salt has already been known. The photothermographic material has an image forming layer including a reducible silver salt (for example, an organic silver salt), a photosensitive silver halide, and if necessary, a toner for controlling the color tone of silver, dispersed in a binder.
A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. There is much literature in which photothermographic materials are described, and the Fuji Medical Dry Imager FM-DP L is a practical example of a medical image forming system using a photothermographic material that has been marketed.
Photothermographic materials utilizing organic silver salts incorporate all essential components in the film and have a characteristic of being able to provide an image by only heat treatment after exposure. However on the other hand, there are many technical problems to be solved.
The first is a problem regarding a color tone of the developed silver image. Because the image is composed of developed silver, the color tone of the developed silver is one of the important characteristics concerning the medical diagnosis quality of a final image. A clear and easy-to-read color tone suitable for diagnosis is preferred. Furthermore, doctors and technicians are accustomed to the images of conventional wet developing silver halide film for medical use. Therefore, similar color tone to that of the conventional wet film is preferable. However, color tone of a developed silver image is readily changed depending on the various thermal development conditions, and color tone may be changed to an unfavorable color when a composition of an image forming layer is changed to adjust the various photographic properties. Therefore, it is a very hard task to satisfy overall photographic properties while maintaining preferable color tone.
Another problem is that, because, in a photothermographic material, all silver halides and all components used for forming an image remain in the film after thermal development, the material has significant problems with regard to degradation of image quality during storage. In particular, even a slight change in color tone can be sensed visually, and this is an important problem, particularly in uses for medical diagnosis, because image quality may affect diagnostic performance.
On the other hand, attempts have also been made at applying the above-mentioned photothermographic material as photosensitive material for photographing. The “photosensitive material for photographing” as used herein means a photosensitive material on which images are recorded by a one-shot exposure through a lens, rather than by writing the image information by a scanning exposure with a laser beam or the like. Conventionally, photosensitive materials for photographing are generally known in the field of wet developing photosensitive materials, and include films for medical use such as direct or indirect radiography films, mammography films and the like, various kinds of photomechanical films used in printing, industrial recording films, films for photographing with general-purpose cameras, and the like. For example, an X-ray photothermographic material coated on both sides using a blue fluorescent intensifying screen, a photothermographic material containing tabular silver iodobromide grains described in Japanese Patent Application Laid-Open (JP-A) No. 59-142539, and a photosensitive material for medical use containing tabular grains that have a high content of silver chloride and have (100) major faces, and that are coated on both sides of a support, which is described in JP-A No. 10-282606, are known. Double-sided coated photothermographic materials are also disclosed in other patent documents. However, these known methods for high sensitivity cause extreme deterioration in quality such as deteriorations in color tone of a developed silver image and image storability, and thus they are not usable for practical applications.